This invention relates to a method for manufacturing molten pig iron in a vertical furnace using steel scrap and iron ore as sources of iron.
In the past, steel scrap or scrap iron (hereunder merely referred to as "scrap") has mostly been used as a raw material in electric furnaces. In the manufacture of steel using a converter, scrap has also been charged into molten pig iron as a coolant.
In recent years, however, there has been a surplus of scrap in the market, and there is a need for a more productive use for scrap. In an electric furnace, it is possible to employ scrap as the sole source of iron for steel making. However, in a converter, it is difficult to use large amounts of scrap, and generally it can constitute a maximum of approximately 20% of the iron source in a converter. Therefore, much research is being carried out in connection with developing techniques for employing large amounts of scrap as a raw material for the manufacture of molten pig iron or molten steel.
Japanese Published Unexamined Patent Application No. 57-198206 discloses a method and apparatus for manufacturing molten pig iron using scrap as a raw material. That method employs a converter equipped with a scrap preheating furnace. Scrap is charged into the converter, which already contains molten pig iron. A carbon-containing material (such as coke) and oxygen are blown into the converter. The carbon-containing material combusts, the heat of combustion melts the scrap, and pig iron is manufactured.
The above-described method for steel making in a converter has been developed so as to use scrap for 100% of its iron source. However, the economic practicality of this method depends on the price of scrap, which is very unstable. When the demand for scrap is high, its price rises, and in this case a method for manufacturing molten pig iron or molten steel using only scrap becomes uneconomical. Furthermore, scrap contains various alloying elements such as Ni, Cr, Cu, Sn, and Zn which are often difficult to remove from a melt during refining.
Another source of iron is iron ore, which is available in large quantities, is stable in price, and does not contain the above-described alloying elements which are difficult to remove from scrap. Therefore, it would be desirable to develop a method for manufacturing iron in a converter using scrap as a primary raw material and using iron ore as a secondary raw material which can be substituted for scrap.
One method which can employ large amounts of iron ore as an iron source in a converter is the smelting reduction method. FIG. 1 schematically illustrates an example of a furnace for use in the smelting reduction method. The illustrated furnace 1 is a vertical furnace such as a converter, and smelting furnace. An oxygen blowing lance 9 is disposed in the center of the furnace 1. Iron ore and coal are charged into the furnace 1 through a charging and discharging port 2 in the top of the furnace 1. In the smelting reduction method, the carbon in the coal which is charged into the furnace 1 is dissolved in a molten iron bath 3. When the iron ore is charged into the molten iron bath 3, it is melted by the molten iron, and the iron oxide (primarily Fe.sub.2 O.sub.3) in the iron ore is reduced by the carbon in the molten iron bath 3. The reactions which take place are expressed by the following formula: EQU Fe.sub.2 O.sub.3 +3C.fwdarw.2Fe+3CO-108,090 Kcal/Kmol of Fe.sub.2 O.sub.3( 1)
This reaction is a liquid phase reaction, so it is much faster than the solid phase reduction which takes place in a blast furnace, so even in a small furnace such as a converter, iron can be reduced highly efficiently.
The reaction expressed by formula (1) is highly endothermic. However, if the CO which is produced in the reaction is combusted in accordance with the following formula, a portion of the energy which is consumed in the reaction can be compensated for: EQU CO+1/20.sub.2 .fwdarw.CO.sub.2 +67,590 Kcal/Kmol of CO (2)
However, the CO.sub.2 gas which is formed by the reaction of formula (2) produces an oxidizing atmosphere inside the furnace, so the oxygen potential of the molten phase is increased, and the molten reduction reaction of formula (1) is impeded. In order to compensate for the heat of reaction and prevent the formation of CO.sub.2 gas, it is necessary to supply a carbon-containing material such as fine coal powder and O.sub.2 gas so as to effect a partial oxidation reaction, such as the one expressed by the following formula: EQU C+1/20.sub.2 .fwdarw.CO+29,400 Kcal/Kmol of C (3)
The heat from the high temperature CO gas which is generated can not be utilized, and the gas is simply discharged from the furnace, so the fuel utilization efficiency is poor.
Thus, in conventional methods for manufacturing pig iron, a processing furnace functions as no more than a melting furnace for scrap, or else it functions as a reducing furnace for an oxidizing reaction at the expense of fuel utilization efficiency, and there is currently no method for the manufacture of pig iron which reduces iron ore with high fuel utilization efficiency and uses scrap and iron ore as iron sources.